CCS and Earthquakes - Anything to Worry About?

by Guest Author on 06/25/12

Note: This
is a cross-posting that originally appeared on the NRDC Switchboard blog June
22, 2012. Guest author is George Peridas with Natural Resources Defense
Council.

A paper (“Perspective”)
published this week by Stanford University professors Mark Zoback and Steven
Gorelick in the Proceedings of the National Academy of Sciences questions the
viability of Carbon Capture & Sequestration (CCS) as a climate mitigation
technology. A comprehensive report on the
potential for seismicity from energy technologies more broadly was also
published this week by the National Research Council (NRC). Zoback and Gorelick
raise some valid issues that should be looked at, but reach sweeping
conclusions without evidence or scientific basis. The NRC report presents a far
more balanced analysis of the situation. For the public, some of the key
questions that need to be answered are:

whether these earthquakes could
have undesirable consequences such as leaks of the injected fluids.

Managing earthquakes caused by human
activity is an issue that deserves more attention than it has received to date.
It can and should be done with today’s tools, but it hasn’t been done
everywhere. The NRC report is timely in that respect, and documents known
earthquakes caused by human activities. None of these have been caused by CCS
projects. The largest seismic event has been caused by an oil/gas extraction
operation, while the more frequent sources are geothermal and waste water
injection projects. No felt earthquakes are known to have been caused by
enhanced oil recovery operations that inject CO2. In most cases, common sense
by operators and regulators could have prevented these events. I agree with the
NRC study on this point: further study and modeling are in order. Even though
smaller earthquakes may not cause any damage, causing them is a profoundly bad
idea. It betrays a lack of scrutiny over project operations, especially since
they are avoidable.

Zoback and Gorelick however appear to
have been causing undue alarm in the media. They state (p. 2) that their “principal
concern is not that injection associated with CCS projects is likely to trigger
large earthquakes; the problem is that even small to moderate earthquakes
threaten the seal integrity of a CO2 repository”. They acknowledge that only
slip on large faults can result in earthquakes large enough to cause damage to
human environments, and that such faults are easily identified and avoided. No
objections on that last point. The potential for slip on existing
faults/fractures and seismicity can and should be taken into account during
site selection. This is routinely done as part of a proper geomechanical
assessment, and Federal Underground Injection Control Program regulations
for geologic sequestration operations require “[i]nformation on the
seismic history including the presence and depth of seismic sources and a
determination that the seismicity would not interfere with containment”.[1]
Large seismic events can be avoided in a straightforward way through proper
siting and operations.

Zoback’s and Gorelick’s arguments
against CCS hinge on the assertion that “[b]ecause laboratory studies show that
just a few millimeters of shear displacement are capable of enhancing fracture
and joint permeability, several centimeters of slip would be capable of
creating a permeable hydraulic pathway that could compromise the seal integrity
of the CO2 reservoir and potentially reach the near surface.” In plain English,
the authors are saying that even a small earthquake can cause CO2 to escape all
the way to the surface, without investigating the circumstances under which
this might happen or their applicability to broad scale CCS. This creates the
impression that it will happen in every case, and is a big logical leap and a
gross simplification, for several reasons.

First, the laboratory studies they cite
were performed on granite, which is extremely unlikely to be used as a sealing
layer, or “caprock” in a real-life sequestration project. Almost certainly, the
caprock will be shale or another low permeability sedimentary rock. The way
that a strong but brittle rock like granite deforms in response to stress is
very different from the way that softer and more ductile shales and other
sedimentary rocks deform, and is therefore not a good analogue.[2]

Second, concluding de facto that joint
and fracture permeability in the caprock(s) would increase in all cases, and
that a pathway would be created that would result in the migration of CO2 to
the surface, is wrong. The degree to which joint and fracture permeability is
increased, if at all, depends on many factors, including rock type, stress
state, and in-filling materials. This is well documented in a large body of
literature on shear-induced behavior of fractures and faults (if you want a
flavor, take a look here[3] for
example). In fact, situations abound where many large faults that exhibit large
slip act as seals and have no effect on permeability. Such is the case in
California and Iran, where trapped oil and gas exists despite frequent large
natural earthquakes. In these areas, in fact, faults themselves have
acted as seals as opposed to pathways for fluid migration, and trapped
hydrocarbons over geologic time. Another well-documented event is the magnitude
6.8 earthquake in Chuetsu, which did not
result in any leaks in the nearby Nagaoka CO2 injection project. Despite
frequent and large natural earthquakes therefore, CO2 and other fluids have
remained trapped in the subsurface.

Additionally, assuming that CO2 will
reach the surface implies that the fault in question extends from the injection
zone to the surface. As the authors themselves note, such a large fault would
be easy to identify and avoid. Even if a fault allows CO2 to migrate out of the
injection zone, many sites also have multiple sealing layers that impede the
motion of fluids to the surface as well as multiple permeable layers that can
act as secondary containers. In fact, studies show that such layered systems
can help prevent fluids from reaching the surface.[4]Assuming
that a pathway will be created all the way to the surface is a huge leap of
logic. Fluids can and do move along faults and fractures – but this does not
mean that the containment “box” has been breached – fluids can simply move
within the “box”, leaving the caprocks intact.

In other words, jumping to the
conclusion that a small induced earthquake would result in surface leakage is
wrong. That’s not to say that it cannot happen, but the problem with the
authors’ assertion is that they then postulate that not enough sites for
sequestration can be found that avoid this scenario to meaningfully deploy CCS
at scale. Although they acknowledge that certain geological settings are
ideally suited to secure sequestration of CO2, such as in the case of the
Sleipner project in Norway (which features a highly porous and permeable
reservoir consisting of weak, poorly cemented sandstone that is laterally
extensive), they then extrapolate that not enough sites like Sleipner can be
found around the U.S. to house the necessary volumes of CO2 to mitigate climate
change. This extrapolation is based on speculation and comes with no scientific
justification. The authors do not study the potential for sites like Sleipner –
i.e. with sufficient porosity and permeability to accommodate injected CO2
without giving rise to unacceptable stresses – to be found around the country.
This can only be done with a rigorous geologic assessment, and there is no
evidence to suggest that such sites cannot be found in sufficient numbers.

Not all sequestration sites need to be
slam-dunk cases with porosity and permeability like Sleipner’s in order to
safely accommodate CO2. Of course – wouldn’t it be nice if things were ideal
everywhere, but a wide range of geological settings can also accommodate CO2
safely without causing unacceptable seismicity risk. The regulation of maximum
allowable pressure, evaluation of seismic risk, and of the conditions in which
transmissive faults would threaten groundwater is central to Federal
regulations under the Underground Injection Control Program. Industry and
regulators should take note, however: even though smaller earthquakes caused by
injection may cause no physical damage or human harm, the public may reject the
idea of CO2 injection if these quakes and perceptible.

Zoback and Gorelick’s assertions were
met with skepticism by expert scientists. Sally Benson (Stanford professor of
Energy Resources Engineering and Director of Stanford's Global Climate and
Energy Project, and Lead Coordinating Author of the Underground Geological
Storage Chapter in the IPCC
Special Report on CCS) said
“of course, you need to pick sites carefully, but finding these kinds of
locations does not seem infeasible”. I think Rob Finley hit the nail on the
head when he compared Zoback
and Gorelick's analysis to early criticisms of the Wright brothers and the
notion at the time that airplanes would never work at scale. Rob is the
principal investigator of the Midwest Geological Sequestration Consortium,
which is now operating a large CO2 injection project in Decatur, Illinois, and
has spent considerable time and money investigating the geology of the Illinois
Basin. Julio Friedmann at Lawrence Livermore National Lab points out that “[b]y
2020, we're going to have somewhere between 15 and 20 projects around the
world. That will be a good time to assess what we've learned and whether [CCS]
can be scaled up more.” The last in the series of international conferences on
the subject attracted 1,500
people. None of them appear to have voiced the seeming impossibilities for
CCS that Zoback and Gorelick describe in their “Perspective”.

Should we therefore be alarmed by the
prospect of CO2 injection in terms of earthquakes? My view is “no” – we should
however be vigilant. Improperly conducted CCS does have the potential to cause
earthquakes, due to the volumes of CO2 injected. But preventing and predicting
these is within our capabilities. Avoiding the large ones is straightforward.
It is worth noting that large natural earthquakes have not compromised the
storage security in natural and man-made sites that trap CO2 and hydrocarbons.
This does not mean, of course, that we should tolerate CCS projects that could
cause earthquakes. Avoiding smaller quakes that may not cause harm but may
alarm the public and local communities will require will careful site operation
and regulation. And that can and must be done. Regulators and prospective
injectors, do your homework.

[2]The
technically minded among you may wish to read on… It is an established concept
in rock mechanics that application of shear stress to a fracture will
result in dilatancy (opening of the fracture). The amount of
dilatancydepends on many factors, including the magnitude of the stress applied
normal to the fracture, the strength of the rock, roughness of the surfaces of
the fracture, and what kind of material is present in the fracture. If a
fracture dilates, its permeability can increase. Granite is at one end of
the spectrum of possible outcomes. It is strong, and fractures are often rough,
so permeability increases can be large. At the other end of the spectrum
are soft shales where dilatancy can be much smaller, or even negligible. Active
faults, which see relative movement over geologic time, are filled with all
sorts of materials representing a spectrum of hydraulic properties. But,
often, they are filled with "gouge", which is essentially clay, which
can sustain large shear movement without large dilatancy.